1. Field of the Invention
The present invention relates to a fire detector and, more particularly, to a photoelectric type of fire detector capable of photoelectrically detecting smoke generated as a result of a fire, as well as to a heat-photoelectric type of fire detector which detects presence of fire by sensing both heat and smoke generated by the fire.
2. Description of the Related Art
A photoelectric type of smoke detector is known from, for example, the disclosure of Japanese Patent Publication No. 63-34520. This device has a housing composed of a detector body and a cover fitting on the body. A printed circuit board is provided on the bottom of the body. An optic base is disposed in an opening formed in the upper side of the housing. The optic base carries a light-emitting element and a light-receiving element arranged such that the optical axes of these elements are nearly parallel with the optic base. A top plate with a labyrinth is placed through the opening in the upper side of the housing so as to cover the optic base. The outer peripheral surface of the top plate is covered with a bug screen.
Assembling and disassembling this known smoke detector requires a great deal of work that is time-consuming due to the use of many screws as fixing means. The optic base in the opening of the housing is held by an L-shaped support member so as to be spaced from the printed circuit board in order to protect electric circuit parts. Consequently, the height of the housing is increased by the height of the L-shaped support member. The thickness of the detector could be reduced by reducing the height of a dark box which is formed on the optic base. This solution, however, may lead to impairment of the performance of the detector. Therefore, the dark box is inevitably designed to have an ordinary height, and the overall thickness of the smoke detector is increased accordingly.
Fixing the L-shaped support member to the printed circuit board also is troublesome. In addition, the support member tends to be deformed, which makes it difficult to fix the optic base at a preselected distance from the printed circuit board. Displacement of the optic base may cause a misalignment of optical axis between the light emitting element and the light receiving element.
In general, a photoelectric type of smoke detector employs a shield case made of, for example, an iron sheet which electrostatically and electromagnetically shields the light receiving element in order to prevent erroneous operation attributable to noise induced by electromagnetic waves or the like. The use of such a shield case, however, increases noise light components because the light emitted from the light-emitting element, as well as the light reflected by the inner surface of the dark box, impinges upon and is reflected by the shield case, thus impairing precision of detection. In order to overcome these problems, the shield case is usually painted black.
The use of a shield case painted black poses the following problems.
A: The shield case is to be soldered to the printed circuit board directly at its legs or indirectly through lead wires. Therefore, the portions where the soldering is done should be left unpainted or, alternatively, the paint on these portions should be removed before soldering. Effecting either of such techniques is troublesome. PA1 B: Assembling the detector requires the greatest care so as not to damage the shield case by, for example, an assembly tool; otherwise, the noise light components increase due to scattering of light by the damaged portion of the shield case where the paint is removed. PA1 C: Dust accumulated in the dark box is to be removed in periodical inspection of the smoke detector. This essentially requires detaching and attaching a lid of the dark box. The cleaning work must be done with the greatest care so as not to damage the paint on the shield case by the lid or a cleaning tool.
In general, a known smoke detector employs, as the light-emitting element, a so-called bullet type of diode having a substantially hemispherical top portion and a pair of lead terminals extending downward from the lower end. This type of diode emits light such as infrared light rays upward or forward from the hemispherical top portion. This type of light-emitting diode is laid in the smoke detector in such a manner as to emit the light substantially parallel with the bottom wall of the dark box or at a certain angle thereto. This essentially requires that the lead terminals of the diode are bent at a certain angle. Consequently, the assembling of this known smoke detector requires a task of bending the lead terminals of the light emitting diode at a certain angle. In addition, an excessive force might possibly be applied to the main part of the diode, when bending the terminals, and cause the diode to be destroyed.
Practically, it is difficult to bend the lead terminals of all light-emitting diodes precisely at the same angle. In other words, the bending angle varies with each individual diode. Such variation in the bending angle causes an offset of the mounting height of the diode when the lead terminals are soldered to the printed circuit board. This leads to mis-alignment of the optical axis of the light-emitting diode with that of the light-receiving element, impairing the precision of smoke detection.
Attempts have been made to obviate the variation in the mounting height of the light emitting diode, such as to insert and fix the light-emitting diode in a diode holding portion provided in the dark box, or to fix the diode on the diode holding portion by means of a retainer plate. Such fixing methods, however, are not recommended because the light-emitting diode could be destroyed or the life of the same shortened due to forces applied to the lead terminals during the fixing.
In general, it is not possible to bend the lead terminals at their base ends. Consequently, the overall length of the light-emitting diode in the state of use is increased, which undesirably impedes reduction in the diameter of the dark box, making it difficult to design and produce a compact smoke detector.
A heat-photoelectric type of fire detector is also known in which the above-described smoke detecting function is combined with a fire detecting function of sensing to heat. This combined type of fire detector employs a heat sensing element projecting to the outside of the housing through an opening in the cover. The lead wires of the heat sensing element are fixed to the top plate having the labyrinth by means of an adhesive tape.
In this type of fire detector, it has been difficult to precisely locate and fix the heat sensing element at a predetermined position, due to the use of the adhesive tape for fixing the lead wires. In addition, the heat sensing element, even when placed at the predetermined position, tends to be shifted if the lead wires are pulled, thus impairing precision of the fire detection.